JP-A No. 2004-104684 discloses a wireless head set device for a motorcycle that enables a user to listen to voice from portable audio equipment provided with a radio communication facility using the wireless head set.
According to a technique disclosed in JP-A No. 2004-104684, as it is difficult for the user to hear/understand voice that flows from the head set in a case that noise (a running sound) caused according to an operational status of the vehicle increases when a piece of music (voice) from the portable audio equipment is being listened to with the head set while a motorcycle is traveling, an operating device mounted in the portable audio equipment, the head set, or elsewhere is required to be operated to vary volume every time the operational status of the vehicle varies, and this is troublesome to a user.